Known in the prior art is a hydraulic apparatus for driving piles (Cf. FRG Patent No. 2,900,221; Cl. E 02 D 7/10), comprising a casing, an impact weight secured on the casing, a hydraulic cylinder housing a pressure-applying piston whose rod is connected with the impact weight, and a hydraulic distributor accomodated in the hydraulic cylinder and constituted by an assembly including a distributing slide valve with a plunger capable of cooperating with the axial-end surface of the power piston. The slide valve and the plunger form a closed volume (a control cylinder) communicating with the space housing the piston, the stroke of which is limited by a stop member. The control cylinder is connected with a drain outlet via a safety valve. The above-cited apparatus is operative and is put into practice, but it suffers from two disadvantages, namely: a high cost because of the need to use a slide valve-type hydraulic control system, and of the need for a precision machining. Besides, hydraulic losses experienced during reversal account for up to 20% of energy of the entire operating cycle due to the so-called "short-circuiting" of the slide valve.
Also known in the prior art is a technical solution in accordance with FRG Patent No. 2,708,512 (E 02 D 7/10) that comes nearest to the present invention and is directed to a pile driving hydraulic hammer comprising a casing, an impact weight mounted so as to be capable of performing a reciprocating motion in relation to the casing, a double-action hydraulic cylinder mounted on the casing and having a piston rod dividing the interior of the hydraulic cylinder into, two cavities, viz. a rod cavity facing a pile, and a piston cavity on the other side of the piston, the piston rod being connected with the impact weight. The above-cited hammer further comprises a pump, a drain-pipe line, a pressure pipe line in permanent communication with the rod cavity, two two-position valves disposed in valve bodies and adapted to alternately put the piston cavity in communication with either the pressure line, or the drain line. These valves are manufactured in accordance with FRG Patent No. 2,654,219 (F 15 B 13/042) in such a manner that each valve would have two control pistons, the diameters of which are smaller than the working diameter of the valve seat. With this structural arrangement of the valves, during their reversal, first an open valve gets closed under the effect of the control piston, while a closed valve gets opened only after equalization of pressure, in the piston cavity and in the pressure line or in the drain one.
The main limitation inherent in the latter-cited hammer resides in the fact that its closed valve is unreliably actuated at the end of an idle stroke, i.e. when the piston and impact weight move upwards. The point is that during an idle stroke an open valve puts the piston cavity in communication with the drain line, while a closed valve separates, the piston cavity the pressure line and, consequently, is blocked in its closed position by the working pressure equal to that in the pressure line. After closure of the open valve, the piston cavity becomes closed and pressure therein is increased only due to kinetic energy of the impact weight which continues to move upwards by inertia until it is, arrested by the braking effect of the impact weight, friction and of the hydraulic force exerted onto the piston in the piston cavity. Thus, to enable the closed valve to get opened, pressure in the piston cavity must reach the working pressure value, i.e. to become equal to that in the pressure line. However, this condition is not always observed, and then the piston and impact weight hang up in their upper position and no working stroke takes place. The reason of this hang-up resides in the fact that frequently kinetic energy of the piston and impact weight proves to be insufficient to build up within the piston cavity a pressure having a required value. For instance, when a hammer is operated not at its full, but only at its partial energy, its stroke is not big and, consequently, its velocity is low. In up-to-date hydraulic hammers operated with a full stroke, their impact weight at the end of an idle stroke is accelerated up to approximately 1.8 m/sec, whereas in operation with minimum blow energy this value is only about 0.3 m/sec, that is in these two cases the kinetic energy of the impact weight this energy is proportional to the square of velocity) differs by 36 times. Besides, a pressure rise in the piston cavity at the end of an idle stroke due to the braking effect of the impact weight is also slowed down by imprevisible leaks from the piston cavity and by the presence of air in the working fluid. This disadvantage of the prior-art hammer is confirmed not only by appropriate calculations of the working process, but also by our own experience: in the hammers we have developed valves were used of a type similar to the described valves and we had to give up, since all our attempts failed to eliminate constant hangs-up of the piston and impact weight in their upper position.
The above-cited hydraulic hammer has yet other drawbacks. For instance, at the end of its working stroke, a valve is first closed through which the piston cavity is put in communication with the pressure line. At the end the valve's stroke, when the slit between the valve and its seat becomes narrow, while its hydraulic resistance becomes, accordingly, great, a working pressure acts upon nearly the entire cross-sectional surface area of the valve (substracting only the control piston surface area), while the backpressure onto the valve from the side of the piston cavity becomes insignificant because of the throttling effect of the slit. As a result, a valve having a weight of the order of 1 kg is accelerated with an effort reaching several tons up to a high velocity and breaks down at it contacts the seat. Since the valve-accelerating pressure acts upon the valve over a great surface area, it is necessary, to effectively brake it, to provide a hydraulic brake with a chamber whose diameter would be comparable to that of the seat, thereby greatly complicating the structure.